Since this will be a rear-hub motor is a torque arm really necessary? Logic would suggest that any pressure from the hub spinning would go inwards on the frame and not backwards where it "could" brake off the dropout?

I completely understand the logic of fitting it for a front-hub motor, but for a rear hub???

With 1000 watts and an aluminum rear dropout, I think most people would recommend using at least one steel torque arm. I use two on my aluminum 1000 watt bike.

The motor axle is only about 14mm in diameter. Given that small radius, the amount of torque applied to the dropouts can be quite high. And compared to the hard steel that the axle is made from, the aluminum used for the dropout is soft. So steel torque arms are generally considered a good idea. The little bit of extra cost to buy them or time to build them is well worth the peace of mind and protection from potential damage that they provide.

Yes. Plenty of people have opened their rear dropouts with hub motors. It may not happen right away on 1000W but you will stress the dropouts every time you apply power. There is not that much metal there and although you have worked out that the axle won't have optimum leverage on the weakest part, you are tempting fate to not run torque arms.

It really sucks having your motor spin out of the dropouts.

For power levels that are actually fun, forget torque arms: a custom rear arm with huge reinforced slots is necessary.

If it was a 250W rear hubmotor and a stout steel frame, I'd say it's OK. Steel would usually start to bend, and possibly give you some warning. Aluminum is more likely to develop a crack, and then suddenly snap off.

You may nor fear a crash at low speed. However, besides the crash there is the repair to the bike and hubmotor to consider. There are pictures of the aftermath available. Spinning the axle will tear out the wires. Three fat phase wires, and five tiny hall sensor wires.

Repairing the wires is not technically difficult, but it is a time-consuming pain in the @$$, which requires disassembly of the motor. Your call, proceed as you wish.

Yes. Plenty of people have opened their rear dropouts with hub motors. It may not happen right away on 1000W but you will stress the dropouts every time you apply power. There is not that much metal there and although you have worked out that the axle won't have optimum leverage on the weakest part, you are tempting fate to not run torque arms.

It really sucks having your motor spin out of the dropouts.

For power levels that are actually fun, forget torque arms: a custom rear arm with huge reinforced slots is necessary.

Wouldn't a 1000 Watt BBSHD apply the same kind of pressure on the dropouts? Just coming from the chain instead?

No. The hub motor applies a twisting force that tries to spread the dropouts. Just because the part on the back is at the top of the dropout, doesn't mean there isn't a twisting force stressing the metal. You know about lb-ft? Your motor probably makes in the ballpark of 20. Leverage is proportional to distance; the edge of the axle is probably 1/4" away from the center. So you are multiplying that 20lbft by 12*4. That's 960 lbs hammering on your wimpy little dropout every time you go full throttle. That's a significant portion of the loadbearing capacity of the dropout. More than it can take without eventual consequences. So the problem is fatigue and then breakage at some point later, but probably not right away.

Wouldn't a 1000 Watt BBSHD apply the same kind of pressure on the dropouts? Just coming from the chain instead?

No. The force is applied from the chain and to the rear cluster. The cluster transfers the force to the hub, spokes and rim. That assembly spins on bearings that ride on the axle. You can't transfer much torque through those bearings considering that the bearings are specifically designed to allow the hub to spin freely. The whole idea is to convert the torque from the chainring to torqe at the rear wheel. It would be counter-productive to be applying that torque to the dropout.

I'm not sure I could explain the engineering reason a mid drive doesn't affect the drop-outs the same way. However, I have been hitting 1500W on my mid drive and the aluminum drop-outs don't have a torque-arm, and show no signs of cracks developing.

Steel drop-outs are stronger than aluminum, and I have seen 1,000W hubmotors ruin steel dropouts when they didn't have a torque-arm.

For the possibility of "what if" at 50 kmh I installed 2 torque arms with the same setup you have

Axles that are hollowed out for cables on one end can't transmit nearly as much torque through that end as they can through the solid end. So a good thick, tightly fitted, well anchored torque arm on the solid end of the axle is not significantly less secure than having one on each end, though having two is more fault tolerant.

This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.